Loaded resonators for adjusting frequency response of acoustic wave resonators

ABSTRACT

An acoustic wave filter device is disclosed. The device includes an acoustic wave filter element, and a first resonator and a second resonator coupled to the acoustic wave filter element. The acoustic wave filter element includes interdigited input electrodes and output electrodes located on a top surface of a piezoelectric layer and an counter-electrode on the bottom surface of the piezoelectric layer. Each of the first and the second resonators includes a resonator electrode on the top surface of the piezoelectric layer and a resonator counter-electrode on the bottom surface of the piezoelectric layer. The first resonator has a first notch in resonator impedance at a first frequency. The second resonator includes a first mass loading layer on the second resonator electrode such that the second resonator has a second notch in resonator impedance at a second frequency that is different from the first frequency.

BACKGROUND Technical Field

This specification relates to thin film radio-frequency acoustic wavefilters.

Background

Radio-frequency (“RF”) components, such as resonators and filters, basedon microacoustic and thin-film technology are widely used in radioapplications such as: mobile phones, wireless networks, satellitepositioning, etc. Their advantages over their lumped-element, ceramic,and electromagnetic counterparts include small size and mass-productioncapability.

SUMMARY

This specification describes technologies for band pass Lateral BulkAcoustic Wave (“LBAW”) filters. More particularly, the presentdisclosure provides techniques to suppress sidebands in LBAW filters andimprove band pass filter characteristic of LBAW filters.

LBAWs can be used as band pass filters. The band pass filter may includeone or more undesired (or parasitic) sidebands. Implementations of thepresent disclosure provide techniques to suppress the undesiredsidebands by adding one or more acoustic resonators in parallel with theLBAW.

LBAW filters are formed from a piezoelectric layer sandwiched betweentwo pairs of electrodes. One electrode from each pair is located on thetop surface of the piezoelectric layer, and forms an input or an outputof the LBAW. The input and output electrodes are separated by a gap.Each pair also has a counter electrode located on the bottom surface ofthe piezoelectric layer. By applying an alternating voltage across thepiezoelectric layer at the input resonator, a mechanical resonance isformed in the piezoelectric layer below the input electrode. Thepiezoelectric layer thickness and the gap between electrodes can bedesigned such that this mechanical resonance is coupled across the gapto the output resonator. The frequency range at which such couplingoccurs determines the achievable bandwidth (or width of passband) forthe LBAW filter.

The foregoing and other embodiments can each optionally include one ormore of the following features, alone or in combination.

In general, one innovative aspect of the subject matter described inthis specification can be embodied in an acoustic wave filter devicethat includes an acoustic wave filter element, a first resonator and asecond resonator. The first resonator and the second resonator arecoupled to the acoustic wave filter element. The acoustic wave filterelement includes interdigited input electrodes and output electrodeslocated on a top surface of a piezoelectric layer and acounter-electrode on the bottom surface of the piezoelectric layer. Thefirst resonator includes a first resonator electrode on the top surfaceof the piezoelectric layer. The first resonator also includes a firstresonator counter-electrode on the bottom surface of the piezoelectriclayer. The first resonator has a first notch in resonator impedance at afirst frequency. The second resonator includes a second resonatorelectrode on the top surface of the piezoelectric layer, and a secondresonator counter-electrode on the bottom surface of the piezoelectriclayer. The second resonator also includes a first mass loading layer onthe second resonator electrode such that the second resonator has asecond notch in resonator impedance at a second frequency that isdifferent from the first frequency.

The first frequency and the second frequency can be within a sideband ofresonator impedance of the acoustic wave filter element. In someexamples, the first frequency and second frequency can differ by atleast 3%.

The resonator electrode can be electrically coupled to the inputelectrodes, and the second resonator electrode is electrically coupledto the output electrodes.

The first resonator electrode can be electrically coupled to the inputor output electrodes, and the second resonator electrode is electricallycoupled to the first resonator electrode. In some embodiments, the firstresonator electrode has a first edge facing the acoustic wave filterelement, the first resonator electrode has a second edge on a side ofthe first resonator electrode farther from the acoustic wave filterelement, and a third edge connecting the first edge and the second edge.In some examples, the second resonator electrode positioned adjacent thesecond edge of the first resonator electrode. In some examples, secondresonator electrode positioned adjacent the third edge of the firstresonator electrode. In some embodiments, the first resonator electrodeand second resonator electrode are electrically coupled by sharing acommon edge. In some embodiments, the first resonator electrode andsecond resonator electrode are electrically coupled by a conductive linethat bridges a gap between the first resonator electrode and secondresonator electrode.

The acoustic wave filter device can include a third resonator coupled tothe acoustic wave filter element. The third resonator can include athird resonator electrode on the top surface of the piezoelectric layer,a third resonator counter-electrode on the bottom surface of thepiezoelectric layer, the first mass loading layer on the third resonatorelectrode, and a second mass loading layer on the third resonatorelectrode such that the third resonator has a third notch in resonatorimpedance at a third frequency that is different from the first andsecond frequencies.

The second mass loading layer can be disposed on the first mass loadinglayer. The first mass loading layer can include a silicon oxide layer.The second mass loading layer can include a silicon nitride layer.

The acoustic wave filter device can include a fourth resonator coupledto the acoustic wave filter element. Thee fourth resonator can include afourth resonator electrode on the top surface of the piezoelectriclayer, and a fourth resonator counter-electrode on the bottom surface ofthe piezoelectric layer. The fourth resonator can have a fourth notch inresonator impedance at the first frequency.

In some embodiments, the first resonator electrode is an uppermost layerof the first resonator. In some embodiments, the first mass loadinglayer does not cover the second resonator electrode.

A first portion of the first mass loading layer can be disposed on thefirst resonator electrode, and a second portion of the first massloading layer can be disposed on the second resonator electrode. Thefirst portion and the second portion can have different thicknesses.

The counter-electrode of the acoustic wave filter element, the firstresonator counter-electrode, and the second resonator counter-electrodecan be provided by a common counter-electrode that continuously spansthe acoustic wave filter element, first resonator and second resonator.

The input electrodes, output electrodes, first resonator electrode, andsecond resonator electrode can be provided by separate portions of thesame electrode layer on the top surface of the piezoelectric layer.

In some embodiments, a thickness of the piezoelectric layer and a gapwidth between the input and output electrodes is such that applicationof a radio frequency voltage between the input electrodes and thecounter electrode will create symmetric and antisymmetric acousticthickness-extensional resonance modes in the piezoelectric layer.

The acoustic wave filter element can be a laterally acoustically coupledbulk acoustic wave (LBAW) filter.

Another innovative aspect of the subject matter described in thisspecification can be embodied in an acoustic wave filter device thatincludes an acoustic wave filter element, a first resonator, and asecond resonator. The first resonator and the second resonator arecoupled to the acoustic wave filter element. The acoustic wave filterelement includes interdigited input electrodes and output electrodeslocated on a top surface of a piezoelectric layer. The first resonatorincludes a first resonator electrode on the top surface of thepiezoelectric layer. The first resonator has a first notch in resonatorimpedance at a first frequency. The second resonator includes a secondresonator electrode on the top surface of the piezoelectric layer. Thesecond resonator also includes a first mass loading layer on the secondresonator electrode such that the second resonator has a second notch inresonator impedance at a second frequency that is different from thefirst frequency.

Another innovative aspect of the subject matter described in thisspecification can be embodied in an acoustic wave filter device thatincludes an acoustic wave filter element, a first resonator, and asecond resonator. The first resonator and the second resonator arecoupled to the acoustic wave filter element. The acoustic wave filterelement includes an electrode located on a top surface of apiezoelectric layer and a counter-electrode located on the bottomsurface of the piezoelectric layer. The first resonator includes a firstresonator electrode on the top surface of the piezoelectric layer and afirst resonator counter-electrode on the bottom surface of thepiezoelectric layer. The first resonator has a first notch in resonatorimpedance at a first frequency. The second resonator includes a secondresonator electrode on the top surface of the piezoelectric layer, asecond resonator counter-electrode on the bottom surface of thepiezoelectric layer. The second resonator also includes a first massloading layer on the second resonator electrode such that the secondresonator has a second notch in resonator impedance at a secondfrequency that is different from the first frequency.

The subject matter described in this specification can be implemented inparticular embodiments so as to realize one or more of the followingadvantages. Band pass filters described herein improve the band passresponse of acoustic filters, e.g., LBAW filters, by suppressingparasitic sidebands. The suppression can be made in particularfrequencies or over a range of frequencies. In addition, LBAW filtersdescribed herein can be simpler to fabricate because they use only asingle piezoelectric layer as compared to two in vertically stacked bulkacoustic wave (BAW) coupled resonator filters. They can also operate athigher frequencies as surface acoustic wave (SAW) filters as theiroperation is determined more by piezoelectric layer thickness thaninterdigital transducer (IDT) electrode dimensions. In some embodiments,LBAW filters can also achieve a wider bandwidth than BAW filters. LBAWfilters can perform as filters with a single lithographic patterningstep as compared to close to 10 in BAW and can operate withoutreflectors needed in SAW, and thus in smaller size.

The details of one or more embodiments of the subject matter of thisspecification are set forth in the accompanying drawings and thedescription below. Other features, aspects, and advantages of thesubject matter will become apparent from the description, the drawings,and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic perspective view of a solidly-mounted LBAWfilter.

FIG. 1B is a schematic perspective view of a self-supported LBAW filter.

FIG. 1C is a schematic planar view of an interdigital transducer (“IDT”)electrode structure.

FIGS. 2A-B are schematic diagrams of two types of propagating plate wavemodes in LBAW piezo layer.

FIG. 3 is a dispersion curves for an exemplary LBAW.

FIG. 4A is a schematic diagram of two resonant modes in an LBAW.

FIG. 4B is an illustrative transmission response of an LBAW as afunction of frequency.

FIG. 5 is an experimental transmission curve of an LBAW as a function offrequency.

FIGS. 6A-B are schematic cross-sectional and planar views respectivelyof a circuit including an LBAW connected to acoustic filter structures.

FIG. 6C is a circuit diagram of the circuit in FIGS. 6A-B.

FIGS. 7A-C illustrate example band pass filters including one or moreacoustic resonators in parallel with an LBAW filter.

FIG. 8 depicts an experimental plot of insertion loss for an LBAW filterwith multiple parallel resonators, and an experimental plot ofimpedances for the parallel resonators.

FIGS. 9A-B depict example resonators integrated to an LBAW filterthrough one or more connections.

FIG. 10 depicts an example of a filter including an LBAW in parallelwith multiple resonators.

Like reference numbers and designations in the various drawings indicatelike elements.

DETAILED DESCRIPTION

FIGS. 1A, 1C show an example of an LBAW filter (or resonator) 100 withinput 150 and output 170 electrodes that have an interdigitated geometry(also called “interdigital transducer” or “IDT” LBAW). LBAW filter 100includes a piezoelectric (“piezo”) layer 110, having a thickness d, anIDT electrode structure 102 located on the top surface of the piezolayer, and a bottom counter electrode 120 located on the bottom surfaceof the piezo layer. IDT electrode structure (“IDT”) 102 includes twocomb-shaped electrodes, 150 and 170, of conductive material, e.g., metalor polysilicon. IDT electrodes 150 and 170 have parallel extensions 150a and 170 a, respectively, that provide the “tines” or “teeth” or“fingers” of the “comb.” Electrode 150 and counter electrode 120 form aninput resonator with piezo layer 110. Electrode 170 and counterelectrode 120 form an output resonator with piezo layer 110.

Acoustic vibrations are created in piezo layer 110 by applying anoscillating (or alternating) input voltage across IDT electrode 150 andbottom counter electrode 120 at an input port 160. The applied voltageis transformed into a mechanical (e.g., acoustic) vibration via thepiezoelectric effect. Under resonance conditions (e.g., with certainacoustic resonance modes, as detailed further below), this vibration cancreate a standing wave under input electrode 150 and an evanescent wave(with exponentially decaying amplitude) in the gap region 190. Withappropriate selection of vibration frequencies and gap width G, thestanding wave can couple mechanically across gap 190 from the piezoregion under electrode 150 to piezo region under electrode 170 andcreate a similar standing wave in piezo layer 110 under electrode 170.The standing wave under electrode 170 results in an output signalvoltage with the same frequency at an output port 180 via the reversepiezoelectric effect. The frequency range at which this coupling occursin mechanical resonance with strong piezoelectric coupling forms thepassband (or bandwidth) of LBAW filter 100. In some example, thefrequency range is between 1.8 and 1.95 GHz. As discussed further below,the thicknesses and geometries, and spacing of the various layers ofLBAW 100 can be tuned to change the RF response and passband of thefilter.

A reflecting structure 130 can serve to isolate the vibration in piezolayer 110 from an underlying substrate 140 and to prevent acousticleakage. Thin layer structure can, for example, be a Bragg reflectorcomposed of alternating high and low acoustic impedance (“Z_(ac)”)material layers. In some embodiments, the thickness of these layers canbe designed such that the frequencies with and near the passband of LBAWfilter are reflected back into piezo layer 110 and all other frequenciespass through the mirror.

In some embodiments, LBAW 100 does not directly overlie substrate 140(as shown in FIG. 1A), but is self-supported, as shown in FIG. 1B. Insuch arrangement, substrate 140 and mirror 130 are replaced by an airgap, with portions of piezo that extend laterally past the region inwhich LBAW 100 is fabricated being supported by substrate 140.

In some embodiments, as shown in FIG. 1C, extensions 150 a and 170 a arerectangular and have a width W, length L, and are spaced by gap width G.Each electrode 150 and 170 has one or more extensions 150 a and 170 arespectively. The total number of electrode extensions is designated asK.

Although FIG. 1C shows rectangular interdigital electrodes 150/170 withparallel extensions 150 a/ 170 a of same geometry and spacing G, otherelectrode geometries are also contemplated. Design considerationsinclude the gap between electrodes, the length of the electrode, and thenumber, if any, and shape of electrode extensions. The gap can be usedto control coupling between the input and output electrodes. Longerelectrodes can also increase coupling. The number of extensions K can beused to control the bandwidth and/or to increase coupling whileconserving the area taken up by the electrodes. In some embodiments, theelectrodes are composed of rectangular strips, with two or moreextensions (e.g., K≥2). For example, each extension can be a rectangularstrip. In some embodiments, the electrodes are concentric circles orspirals having a common axis.

Piezo layer 110 can be formed from various piezoelectric materials.Exemplary materials include ZnO, AlN, CdS, PZT, LiNbO₃, LiTaO₃, quartz,KNN, BST, GaN, Sc alloyed AlN, or the aforementioned materials doped oralloyed with an additional element. Doping can be used to improve ortailor electromechanical properties of piezo layer 110. As detailedfurther below, piezo layer thickness d is selected such thatthickness-extensional modes near the frequencies of the desiredbandwidth of the LBAW filter are produced in the piezo layer. In someembodiments, piezo layer thickness d is 20% to 50% of λ_(z), or 30% to45% of λ_(z), where λ_(z) is the wavelength of the piezoelectricvibration in the thickness direction. In some embodiments, d is 1500 nmto 2500 nm, or 1800 to 2200 nm.

Thin film IDT 102 can be composed of various materials. In someembodiments, IDT electrodes 150 and 170 are metal. For example, theelectrode material includes Al, Mo, Pt, Cu, Au, Ag, Ti, W, Ir, Ru, ormultilayers of metals and/or metals doped with additional materials,e.g. AlSi, AlSiCu, polysilicon, etc. Doping can be used to improve ortailor IDT electric or mechanical properties.

Although FIG. 1A shows a single common counter electrode 120, filter 100can include separate electrodes for the input and output resonators.Various materials are suitable for the counter electrode(s) (e.g.,electrode 120). For example, the electrodes can include a metal, such asAl, Mo, Pt, Cu, Au, Ag, Ti, W, Ir, Ru, or multilayers of metals and/ormetals doped with additional materials, e.g. AlSi, AlSiCu etc. Dopingcan be used to improve or tailor IDT electric or mechanical properties.For example, the electrodes can be Ti+Mo, Ti+W, AlN+Mo, or Al+W. Theelectrodes can be multilayered. The electrodes can have a special thinseed layer deposited below the electrode.

Reflecting structure 130 can be composed of alternating layers ofdifferent materials. For example, reflecting structure 130 can includealternating layers of two of: Tungsten (W), SiO₂, silicon (Si), carbon(C). For example, layers of high acoustic impedance include be W, Mo,Ir, Al₂O₃, diamond, Pt, AlN, Si₃N₄. Layers of low acoustic impedance caninclude SiO₂, glass, Al, Ti, C, polymers, or porous materials. Layer ofSi provides an intermediate acoustic impedance. Various materials aresuitable for the substrate 140, such as Si or SiO₂ or glass, sapphire,quartz. Substrate 140 materials can have high electrical resistivity.The substrate can have a thickness appropriate for RF applications, suchas integration into mobile phone platforms. For example, the substratecan have a thickness less than 500 microns, or less than 200 microns.For example, Si wafers can be purchased with a thickness of 675 μm andthinned down to achieve a desired device thickness, e.g., for mobileplatforms.

Modeling of the acoustic response of LBAW 100 can provide guidance onhow to tune the design parameters for individual elements of thestructure to achieve desired bandpass properties. For example, LBAW 100can be designed to have resonance modes at specific frequencies. Ingeneral, the geometry of various LBAW 100 components can be selected toachieve various acoustic properties. LBAW 100 properties can depend onthe combination of these geometries, which may not be independent of oneanother.

In piezoelectric layer 110, different bulk acoustic vibration modes canarise at different excitation frequencies ƒ of input voltage (e.g., atport 160). Acoustic vibrations in piezo layer 110 can propagatelaterally as Lamb waves (or plate waves), wherein particle motion liesin the plane that contains the direction of wave propagation and theplate normal (e.g., the z-axis in FIG. 1A). Two such modes are shown inFIGS. 2A-2B. Referring to FIG. 2A, a thickness-extensional (TE orlongitudinal) bulk mode 200 has particle displacement 210 dominantlyperpendicular to the propagation direction (in the z-direction).Referring to FIG. 2B, a second order thickness-shear (TS2) bulk mode 220has particle displacement 230 dominantly parallel to the propagationdirection (in the y-direction). For both modes, the lowest frequency atwhich resonance in the thickness direction can arise is when thethickness d of piezo layer 110 is equal to an integer number of halfwavelengths λ_(z) (disregarding the thickness of electrodes 150/170); inother words, when

${d = \frac{N\; \lambda_{z}}{2}},$

where N is an integer that indicates the order of the resonance. For theTE1 mode,

$d = {\frac{\lambda_{z}}{2}.}$

As discussed further below, the width W of the electrodes and the gap Gbetween electrodes can be designed such that TE1 mode standing waveswith certain lateral wavelengths λ_(∥) are formed that can couplethrough their evanescent tails across gap G to create two mechanicalresonant modes.

Acoustic properties of an LBAW resonator 100 can be described withdispersion curves. Referring to FIG. 3, an example dispersion curve foran LBAW 100 shows the lateral wave number k_(∥) of the vibration, where

${k_{} = \frac{2\; \pi}{\lambda_{}}},$

as a function of voltage input frequency ƒ. The first-order longitudinal(thickness extensional, TE1) vibration mode, in which the combinedthickness of the piezoelectric layer d and the thickness of electrode(s)150 or 170 contains approximately half a wavelength of the bulkvibration, λ_(z)/2, and the second-order thickness shear (TS2) mode, inwhich the bulk vibration is dominantly perpendicular to the thicknessdirection (z-axis in FIG. 2B) and one acoustic wavelength λ_(z) iscontained in the combined piezoelectric layer thickness d and thethickness of electrode(s) 150 and 170, are denoted in the figure. TheTE1 mode is the darker portion of each dispersion curve, and TS2 mode isthe lighter region of each dispersion curve. The top curve (“noelectrode”) represents the dispersion properties of the piezoelectriclayer under the gap 190. The bottom curve (“electrode”) represents thedispersion properties of the piezoelectric layer under electrodes150/170, also known as the active region. More specifically, where the“electrode” curve intersects k=0, the TE1 mode has approximately λ_(z)/2contained in the combined thickness of the electrodes 150 or 170 and thepiezoelectric layer. This is approximate because the wave can extendinto the Bragg reflector. “No Electrode” curve intersection with k=0lines shows the modes where approximately λ_(z)/2 is contained in thecombined thickness of the bottom electrode only and the piezolayer. Thistype of dispersion, in which the TE1 mode has increasing k_(∥) withincreasing frequency ƒ, is called Type 1. The difference in intersectk∥=0 frequencies between electrode and non-electrode areas determinedthe hard limits for the achievable bandwidth of the filter. The gapwidth G, electrode width W, and number of extensions K can be used tovary the coupling strength within the limits set by the dispersiondifference.

In some embodiments, LBAW 100 can be designed to produce Type 1dispersion. For example, piezo layer 100 materials can be selected inwhich Type 1 dispersion can occur. For example, ZnO can be used. Inanother example, appropriate design of acoustic Bragg reflector 130 canhelp achieve Type 1 dispersion. For example, using Aluminum nitride(“AIN”) for piezo layer 110 can typically produce a Type 2 dispersion,where TE1 mode behaves non-monotonically having initially decreasingk_(∥) with increasing frequency ƒ, and then increasing k_(∥) withincreasing frequency ƒ, (roughly similar to what is described in thedispersion curves of in FIG. 3 but with TE1 and TS2 interchanged).However, in some embodiments, with an appropriate design of thereflecting structure 130 (e.g., acoustic Bragg reflectors), the LBAW 100can use AIN in piezo layer 100 and still achieve a Type 1 dispersion.See for example Fattinger et al. “Optimization of acoustic dispersionfor high performance thin film BAW resonators,” Proc. IEEE InternationalUltrasonics Symposium, 2005, pp. 1175-1178.

In FIG. 3, positive values of k_(|) denote real wave numbers(propagating waves) and negative k_(∥) values correspond to imaginarywave numbers (evanescent waves). For a resonance to arise, the acousticenergy must be trapped inside the LBAW resonator structure. In thethickness (z-axis) direction, isolation from the substrate (usingreflecting structure 130) can be used for energy trapping. In thelateral direction, energy trapping can occur when an evanescent waveforms outside the electrode region (e.g., on the “no electrode” curve).To get resonant coupling between the two resonators (e.g., electrodes150/170 and 120) of an LBAW, standing waves of a TE1 mode form in theactive regions of the piezo layer (under the electrodes), and evanescentwaves form in the “no electrode” region. In other words, k_(∥) ispositive for the TE1 “electrode” curve and negative for the TE1 “noelectrode” curve. According to FIG. 3, this occurs in the labeled“trapping range” frequency range. Energy trapping can be easier torealize in Type I dispersion. Without wishing to be bound by theory,with monotonically increasing dispersion curves as the thick TE1 linesin FIG. 3, for the “Electrode”, at a single frequency in the trappingrange there is either a single imaginary wave number available or abovethe trapping range a single real wave number. The former means that theTE1 does not propagate outside the electrode, and the latter that theTE1 can couple to a propagating wave outside the electrode and thus“leak”. The Type 2 dispersion can be described by similar curves butwith the TE1 and TS2 curves interchanged. The fact that the curve inType 2 is non-monotonic means that at a given frequency there may beseveral real wavenumbers. Having several wavenumbers for a frequencymeans propagating waves are available outside the electrode, which cancause a “leak”.

FIGS. 4A-4B illustrate the relationship between standing wave resonancemodes and the LBAW bandgap. Referring to FIG. 4A, a portion of LBAW 100includes two adjacent electrodes 401 and 402 with width W (e.g.,corresponding to extensions 150 a and 170 a of respective electrodes 150and 170 of FIG. 1A). The bandpass frequency response of LBAW 100 isformed by two (or more) laterally standing resonance modes, 410 and 420,arising in the structure. Lateral standing wave resonance can arise whenplate waves are reflected from edges of electrodes 401 and 402. In theeven mode resonance 410, the piezo layer under both electrodes 150 and170 vibrates in-phase, whereas in the odd mode resonance 420, the phasesare opposite. The even lateral standing wave resonance can arise whenthe total width of the structure is roughly equal to half of the lateralwavelength λ_(∥) of the mode:

$\frac{\lambda_{even}}{2} = {\frac{\lambda_{}}{2} \approx {{2 \cdot W} + {G.}}}$

In the limit of infinitely small gap width G, λ_(even) approaches thetotal width from below. As shown in FIG. 4A, λ_(even) gets smaller whenG gets larger and gets larger when G gets larger. In case of small gap(e.g., zero-gap) λ_(even) gets close to 4W and in case of a large gapλ_(even) gets close to 2W. The odd lateral standing wave resonance canarise when the width of the electrode is roughly equal to half of thelateral wavelength λ_(∥) of the mode:

$\frac{\lambda_{odd}}{2} = {\frac{\lambda_{}}{2} \approx {W.}}$

Referring to FIG. 4B, the even 410 and odd 420 modes are shown astransmission peaks as a function of input frequency ƒ for an LBAW withType 1 dispersion. For Type 1 dispersion, even mode 410 has a longerwavelength and is lower in frequency than the shorter-wavelength oddmode 420. The frequency difference 430 between the modes determines theachievable bandwidth of LBAW filter 100, and depends on the acousticproperties of the structure and on the dimensions of IDT resonator 102.Acoustic coupling strength can be defined in terms of the (resonance)frequency difference between even (symmetric) and odd (antisymmetric)resonances:

$\frac{f_{asymm} - f_{symm}}{f_{0}},$

where ƒ_(symm) and ƒ_(asymm) are the symmetric and antisymmetriceigenfrequencies, respectively, and ƒ₀=(ƒ_(symm)+ƒ_(asymm))/2 is thecenter frequency between the two modes.

In some embodiments, increasing the number of extensions (e.g., 150 aand 170 a) in each electrode (e.g., 150 and 170) can increase thefrequency difference between the even and odd mode in the LBAW, and thusincrease the bandwidth. This effect can result from the fact that thelateral wavelength of the odd mode can depend on the periodicity of theelectrode structure (e.g., width W), while the even mode can depend onthe entire width of the structure (e.g., adding up all widths W and gapsG). For example, if the total number of electrode extensions K, theelectrode width is W, and the gap width is G, the wavelength λ_(∥) ofthe lateral acoustic wave at the even mode resonance frequencyapproaches or is slightly shorter than:

$\frac{\lambda_{even}}{2} \approx {{K \cdot W} + {K \cdot {G.}}}$

The odd lateral standing wave resonance in this structure, however,approaches or is slightly larger than:

$\frac{\lambda_{odd}}{2} \approx {W.}$

Additionally, or alternatively, in some embodiments, the total width ofthe structure K·W+K·G can be such that the highest-order mode trapped inthe structure is the desired odd mode resonance. For example, K can be31, W can be 3 μm, and G can be 2 μm.

In some embodiments, the number of electrode extensions K is between 2and 200, or between 10 and 60. In some embodiments, the length L ofelectrode extensions can be between 50 μm and 2000 μm, or between 70 μmand 500 μm.

In some embodiments, the gap G is selected to allow coupling of theevanescent tails of standing waves formed under electrodes 150 and 170.For example, the gap G between electrode extensions can be 0.1 μm and 10μm, or between 2 μm and 5 μm.

In some embodiments, electrode 150 and 170 topology can be designed suchthat the gap width G provides good enough coupling between electrodeextensions to create a single even mode 410 across the entire width ofthe structure. For example, the gap width G can be 2%-300%, or 10%-100%of the evanescent acoustic wave's decay length, i.e. the length at whichamplitude A=A₀·e⁻¹ of the original amplitude A_(o), in the gap at thedesired even resonance mode. Gap with G can be optimized. Decreasing thegap to a too small width (1) can eventually pull the even and odd modestoo far from each other creating a dip in the passband, (2) can lead toreduced coupling coefficient for the odd mode, or (3) can increasecapacitive feedthrough from finger to finger causing poor out of bandattenuation.

In some embodiments, gap width G can be defined with respect to piezolayer thickness d. For example, G can be designed to be 10% to 300% ofd, or 25% to 150% of d.

In some embodiments, the width of electrode extensions W can be between0.1 μm and 30 μm, or between 2 μm and 5 μm. In some embodiments, W canbe designed such that the wavelength λ_(∥) of the lateral acoustic waveat the desired odd mode resonance frequency λ_(odd) is obtained.

In some embodiments, electrode width W is designed such that multiplehalf-wavelengths cannot fit within the electrode width. For example, Wcan be designed to be smaller than the lateral acoustic wave'swavelength λ_(∥) at the desired odd resonance mode, e.g., whereλ_(∥)=λ_(odd).

In some embodiments, the thicknesses of various LBAW 100 components canbe selected to achieve various acoustic properties and may beinterdependent. For example, the piezo layer 110 thickness d (minimumand maximum value) can first be determined with respect to the acousticwavelength in the piezo material (λ) at the operation frequency ƒ. Insome embodiments, thicknesses (min and max) of the other LBAW 100 layerscan be selected based on the choice of piezo thickness d. For example,the combined thickness of the electrodes (including the counterelectrode 120) and the piezoelectric layer can be selected to beapproximately half a wavelength of the mode that is being used, forexample longitudinal bulk wave for the thickness extensional mode.Fundamental modes with N=1 (the first mode, i.e., first harmonic) canallow for greater coupling, but N>1 modes are also possible. Forexample, the thickness of electrodes 150 and 170, bottom electrode 120,and reflecting structure 130 can be defined as a percentage of piezolayer thickness d. In some embodiments, once all thickness are selected,the geometry of the electrode extensions 150 a and 170 a, such as numberK, width W, gap G, and length L, can be tuned to match the LBAW 100electrical impedance to the system impedance. Without wishing to bebound by theory, impedance matching can help avoid losses andreflections in the system.

In some embodiments, thickness of electrodes 150 and 170 is between 1%to 30% of d, or 2% to 25% of d, or 3% to 15% of d.

In some embodiments, the thickness of bottom electrode 120 is between 5%to 50% of d, or 10% to 30% of d, or 10% to 20% of d.

In some embodiments, where the reflecting structure 130 is a Braggreflector, the alternative layers of the reflector can be designed suchthat the required reflectivity of passband wavelengths is obtained. Forexample, the thickness of each layer can be equal to or less or morethan one quarter of the acoustic wavelength λ_(z) in the thicknessdirection to reflect the odd and even TE1 resonance modes. In someembodiments, a single layer in the Bragg reflector can be 15% to 80% ofd, or 20% to 70% of d.

The mass loading of the IDT 102, determined by the thickness andmaterial of electrodes 150 and 170, can be designed such that thefrequency difference between the k_(∥)=0 frequency of the electroderegion's TE1 mode and the outside electrode region's TS2 mode is small.Without wishing to be bound by any particular theory, when the frequencydifference between outside region's TS2 mode and electrode region's TE1mode is small, the trapping range is large. More particularly, thek_(∥)=0 frequency of the outside region's TS2 mode can be 95%-99% of theelectrode region's TE1 cutoff frequency. The frequency differencebetween the outside region's TS2 and outside region's TE1 modes' k_(∥)=0frequencies is designed to be large, e.g. 5%-15%, for example 6.5%-7.5%,of the electrode region's TE1 mode cutoff frequency.

According to certain embodiments of the present invention, the k_(∥)=0frequency of the outside region's TS2 mode is greater than, or equal to98%, or between 98% and 99.5%, or is 98.9% of the electrode region's TE1cutoff frequency. Similarly, the frequency distance expressed as thefrequency difference between electrode region TE1 and outside region TS2k_(∥)=0 frequencies:

$\frac{{{electrode}\mspace{14mu} {TE}\; 1} - {{outside}\mspace{11mu} {TS}\; 2}}{{outside}\mspace{11mu} {TS}\; 2}$

should be small, for example on the order of 1%. As an example, saidfrequency distance can be between 0.2% and 2.1%, or between 0.5% and1.8%, or between 0.8% and 1.5%, or for example, 1.1%.

FIG. 5 shows a curve of insertion loss IL (in decibels) versus frequencyƒ for an exemplary LBAW 100. The curve shows two passbands with peak 510corresponding to TE1 waves and peak 520 corresponding to TS2 waves. Asdiscussed above, the width of each passband is determined by thefrequency difference of the even and odd modes for the respective typeof wave. Here, the TS2 modes correspond to sideband 520 a (also referredto herein as “TS2 passband”), and the TE1 modes correspond to passband510 a (also referred to herein as “TE1 passband”). In some embodiments,LBAW 100 is designed to suppress peak 520 corresponding to TS2 modes,while maintaining the properties of peak 510 corresponding to TE1 modes.Without wishing to be bound by any particular theory, TE1 mode operationcan be selected because piezo thin film materials have electromechanicalcoupling that is stronger in the thickness direction. In other words,TE1 longitudinal mode vibrations couple more efficiently to theelectrical excitation over the thickness of piezo layer 110.

In some embodiments, LBAW 100 can be designed to have a passband for TE1modes between 0.5 and 10 GHz, or between 1 and 4 GHz. In some examples,TE1 passband is between 1.8 and 3.7 GHz. The limits of the passband canincorporate design considerations. For example, the dimensions of thedevice can grow very large or very small. Too large dimensions may taketoo much space and cause inefficiencies. Too small dimensions candeteriorate performance due to thin and narrow electrodes leading toresistance and losses. In some embodiments, LBAW 100 can be designed tohave a TE1 passband width 510 a of 0.5-15% relative to center frequency,e.g., 10% relative to center frequency, or 5%, or 2%, or 1%. In someembodiments, the insertion loss at the passband is better than −7 dB,e.g., −7 dB to −0.5 dB or −5 dB to −0.5 dB.

LBAWs can be used as bandpass filters. The IDT electrodes of an LBAW canbe designed to couple a driving electrical signal to a desiredfundamental TE1 mode. An effective coupling results in a passbandsimilar to the TE1 passband of 510 a in FIG. 5. However, the couplingalso results in one or more sidebands similar to the sideband 520 a. Thesideband 520 a can be at a lower frequency than passband 510 a, and canbe narrower than the passband 510 a. The sideband 520 a is generatedbecause of electric field between the parallel extensions (e.g., theextensions 150 a and 170 a) of LBAW. The extensions cause anasymmetrical electric field in the thickness direction of the piezo, andthe asymmetrical electric field couples to both the TE1 and TS2 modes.

Implementations of the present disclosure provide techniques to suppressLBAW sidebands created by TS2 modes. The implementations suppress thesidebands by connecting acoustic resonators to the LBAW. At least one ofthe acoustic resonators has an impedance notch at a resonance frequencywithin the bandwidth of the sideband. As explained in further detailsbelow, the impedance notch causes an increase in insertion loss of theLBAW at the resonance frequency, and results in an overall increase ofthe insertion loss in the sideband.

The acoustic resonators can be added in series or in parallel with LBAW100. For example, referring to FIGS. 6A-B, cross-sectional and planarviews of structure 600 include LBAW 100 connected to resonators 612,613, 615, and 616. FIG. 6C shows the corresponding circuit diagram ofstructure 600. In structure 600, piezo layer 110 is common to LBAW 100and all connected filters. Moving from left to right of FIG. 6B,parallel resonator 612 and series resonator 613 are located before inputport 160 of LBAW 100. Series resonator 615 and parallel resonator 616are located after output port 180 of LBAW 100. In parallel resonators612, 616, the lower electrode is grounded. In series resonators 613,615, the signal goes to the lower non-grounded electrode across piezolayer 110.

Embodiments with one or more series resonators can be designed so thatresonance frequencies of the series resonators fall within passbandfrequencies of the sideband, and suppress the sideband. An acousticresonator (e.g., a BAW/FBAR resonator) has a very high impedance at itsanti-resonance frequency. Such high impedance prevents the drivingelectrical signal to pass through and reduces signal transmission thoughthe filter. Thus, a resonator with one or more anti-resonancefrequencies within sideband of an LBAW filter can be added in series tothe LBAW filter to reduce signal transmission at the one or moreresonance frequencies (which would be parallel frequencies with theLBAW) and suppress LBAW's sideband.

Embodiments with one or more parallel resonators can be designed so thatresonance frequencies of the parallel resonators fall within passbandfrequencies of the sideband, and suppress the sideband. An acousticresonator (e.g., a BAW/FBAR resonator) has a very low impedance at itsresonance frequency. Such low impedance shunts the driving electricalsignal to ground and reduces signal transmission though the filter.Thus, a resonator with one or more resonance frequencies within sidebandof an LBAW filter can be added in parallel to the LBAW filter to reducesignal transmission at the one or more resonance frequencies (whichwould be series frequencies with the LBAW) and suppress LBAW's sideband.In general, one or more parallel resonators can be integrated to an LBAWfilter by (i) using input or output electrodes of the LBAW as parallelresonators (e.g., similar to FIGS. 7A-C), and/or by (ii) connecting theone or more parallel resonators to the LBAW filter (e.g., FIGS. 9A-B).

The frequency of a resonator can be tuned by selecting the mass loadingof the resonator. The mass loading can be achieved by applying one ormore mass loading layers on one or both electrodes of the resonator. Themass loading layer can be composed of a different from or the samematerial as the underlying electrode. In the latter case, the tworesonators can be considered to have electrodes of different thickness.

Different mass loading between two resonators can be achieved by i)applying a mass loading layer on an electrode of one resonator but noton the electrode of the other resonator, ii) applying differentthicknesses of the same material on the two respective electrodes of thetwo resonators, and/or iii) by applying layers of different materials onthe two respective electrodes of the two resonators. In addition,different mass loading between two resonators can be achieved by havingthe electrodes of the two resonators have different thicknesses.

Between two resonators with mass loading layers composed of the samematerial, the resonator with a thicker mass loading layer has a greatermass and thus a lower resonance frequency than the resonator with athinner electrode. Thus, the resonance frequency can be adjusted byadding or removing loads on top of the resonator's electrode.

Parallel resonators (i.e., resonators in parallel with an LBAW) can bedesigned to have resonance frequencies within the frequency range of oneor more sidebands that are to be suppressed (e.g., the frequency rangeof sideband 520 a). Moreover, multiple parallel resonators withdifferent resonance frequencies can be designed to suppress a sidebandover a wider range of frequencies (as compared to parallel resonatorswith the same resonance frequency), or to suppress multiple sidebands.

FIGS. 7A-C illustrate example band pass filters including acousticresonators in parallel with LBAW 100. FIGS. 7A-C illustrate top viewsand cross-sectional views of band pass filters 700, 710, and 720,respectively. Filters 700, 710, and 720 can generally be the same,except that they may differ in one or more mass loading layers in one ormore of their parallel resonators (also referred to herein as“resonators”). The filters 700, 710, 720 can be generally the same asthe filter assembly 600, except as described. For example, the filters700, 710, 720 can optionally omit the series resonators 613, 615,although such series resonators could still be included between the LBAWfilter.

Filter 700 includes parallel resonators 702, 704, filter 710 includesparallel resonators 702, 712, and filter 720 includes parallelresonators 722, 724, 726, and 728. Each of resonators 702, 704, 712,722, 724, 726, and 728 has a resonator electrode (also referred toherein as “electrode”) and a resonator counter electrode. The resonatorelectrode and counter electrode can be used for applying drivingelectrical signal. One or more resonators can have a common electrodewith LBAW 100 (e.g., electrode 150 or 170) such that extensions of LBAW100 are projected from the common electrode. One or more of theresonators can have a common counter electrode with LBAW 100 (e.g.,counter electrode 120). In some embodiment, the common counter electrodecontinuously spans the LBAW and the respective resonator that shares thecommon counter electrode with the LBAW. In some examples, the commoncounter electrode is grounded. Each of the parallel resonators 702, 704,712, 722, 724, 726, and 728 can be made of the same or differentmaterials as the LBAW 100. The parallel resonators can be composed ofpolysilicon, metal, silicon dioxide, or silicon nitride.

Referring to FIG. 7A, the filter 700 includes two parallel resonators702, 704 that are coupled to the input and output electrodes,respectively, of LBAW 100. In particular, each parallel resonator 702,704 includes a respective conductive layer 732, 734 as a resonatorelectrode, and a counter electrode that together sandwich the piezolayer 110. Each conductive layer 732, 734 is electrically coupled to theextensions 716 of the respective input and output electrodes. Inparticular, each of the conductive layers 732, 734 can provide a commonelectrode (e.g., electrode 150, 170 in FIG. 1A) from which therespective extensions 716 (e.g., extensions 150 a, 170 a) project forthe respective input and output electrodes. The common electrode iscommon between the LBAW 100 and the respective parallel resonator. Forexample, one or both of the conductive layers 732, 734, and the LBAWextensions can be provided by separate portions of the same electrodelayer on a top surface of the piezoelectric layer. The parallelresonator 702 and/or 704 can be electrically coupled to the input and/orthe output electrodes or ports (e.g., ports 160, 180) of LBAW 100. Theresonators 702, 704 and the LBAW 100 share a common counter electrode120, e.g., the counter-electrode for each resonators 702, 704 can beprovided by a respective portion of the counter electrode 120. Theresonators 702, 704 and the LBAW 100 also share a common piezo layer110. The counter electrode 120 can be grounded.

As previously explained, resonance frequency of either or both parallelresonators 702 and 704 can be tuned by adding mass loads to, or removingmass loads from the respective conductive layers 732, 734. To reduceresonance frequency of resonator 702, 704, a layer that provides a massload can be deposited on top of the conductive layer 732, 734 of therespective resonator. To increase resonance frequency of resonator 702,704, the conductive layer 732, 734 of the respective resonator can bepartly removed, for example, through etching.

FIG. 7B illustrates a band pass filter 710 with parallel resonator 702and a parallel resonator 712. In particular, the mass loading of the tworesonators 702, 712 is different such that the two resonators 702, 704have different resonant frequencies. Both resonant frequencies can bewithin the frequency range of the sideband 520 of LBAW 100.

The parallel resonator 712 is formed by adding a mass loading layer 708on top of the conductive layer 734 (or electrode) of the resonator 704.In some embodiments the conductive layer 734 is composed of Aluminum(Al) or Copper (Cu), and the layer 708 is composed of silicon oxide(SiO₂) and/or silicon nitride (SiN). Due to the addition of the massloading layer 708, resonator 712 can have a lower frequency than theresonator 704.

The thickness of the layer 708 can be selected to provide a desiredresonance frequency for the resonator 712. For example, if the resonancefrequency of resonator 704 would otherwise be higher than the frequencyrange of the sideband 520 a, the thickness of layer 708 can be selectedso that the resonance frequency shifts, to provide a resonance frequencywithin the sideband 520 a for the resonator 712. The thickness can alsobe adjusted by further thickening (e.g., by depositing more material) orthinning (e.g., by etching) layer 708.

The resonator 702 can have a different thickness than the resonator 712.This thickness difference can result from different thickness of the twoconductive layers 732, 734, or from a mass loading layer being presentin the resonator 712 and absent in the resonator 702, or from differentthicknesses of mass loading layers on the two conductive layers 732,734.

Resonator 712 can be composed of one or more materials that are notincluded in resonator 702. For example, the mass loading layer 708electrode of the resonator 702 may be composed of a material (e.g.,silicon oxide, silicon nitride, etc.) different from the material of theconductive layers 732, 734 (e.g., aluminum, copper, etc.).

The layer 708 can be composed of the same material or different materialthan the conductive layer 734. For example, the conductor layer 734 canbe composed of aluminum (Al), and the layer 708 can be composed of Al,silicon oxide and/or silicon nitride. Where the layer 708 is aconductor, e.g., the same conductive material as conductive layer 734,the resonator 712 can be considered to have a thicker electrode than theresonator 702.

Although FIG. 7B illustrates achieving different mass loading byincluding a mass loading layer 708 in resonator 712 and not including amass loading layer in resonator 702, other techniques are possible. Forexample, both resonators can include mass loading layers of differentthickness and/or material. For example, the mass loading layers of thetwo resonators 702, 704 can be composed of the same material butdifferent thickness. As another example, the mass loading layers of thetwo resonators 702, 704 can be composed of different materials,optionally with the same thickness. Alternatively or in addition, athickness of the resonator electrode (i.e., the conductor layer 732) ofresonator 702 can differ from the thickness of the electrode (i.e., theconductive layer 734) of resonator 712.

As noted previously, multiple resonators with different resonancefrequencies within the sideband can be connected to an LBAW in parallel,to suppress a wider range of frequencies (as compared to a configurationwith a single parallel resonator, or a single resonance frequency) ofthe LBAW's sideband. Multiple parallel resonators with differentresonance frequencies shunt the driving electrical signal to ground ateach of the resonance frequencies. For example, when the two resonators702 and 712 of filter 710 have different resonance frequencies that fallwithin sideband frequencies of the LBAW 100 (e.g., sideband 520 a), thesideband is suppressed over a wider range of frequencies compared towhen the two resonators have the same resonance frequencies.

FIG. 8 depicts an experimental plot 800 of insertion loss for an LBAWfilter with two parallel resonators, and a simulated plot 810 ofimpedances for the two parallel resonators. For example, the tworesonators can be the resonators 722, 728 in FIG. 7C. As depicted inplot 810, the two resonators have two different resonance frequencies804 and 808. The solid line in plot 800 corresponds to insertion loss ofan LBAW. The dashed line in plot 800 depicts insertion loss of the sameLBAW connected in parallel with the two resonators, whose impedances aredepicted in plot 810. For example, the solid line in plot 800 canrepresent insertion loss of LBAW 100, and the dashed line can representLBAW 100 in parallel with resonators 702 and 712, as shown in FIG. 7B.

As illustrated in plot 800, low impedances of the two parallelresonators at their resonance frequencies reduce transmission throughthe filter and cause two dips 802 in sideband 520 a at the resonancefrequencies 804 and 808 of the parallel resonators. The two dips reducethe overall insertion loss in the sideband and also suppress thesideband's peak. The two resonance frequencies can have a frequencydifference of about 0.1 to 3% relative to their center frequency. Eitherof these two resonance frequencies can have a frequency difference of atleast 3%, e.g., at least 10%, with respect to the frequency of the skirtedge 806.

In addition to suppressing the sideband, one or more parallel resonatorscan be added to an LBAW to modify steepness of passband edge of theLBAW. A parallel resonator with resonance frequency at the edge of thepassband provides a sharp dip at the edge and causes a steeper skirt forthe TE1 passband. Further details about specific filter arrangements andthe effect on passband edges are disclosed in U.S. Pat. No. 9,893,712,incorporated herein in its entirety by reference. For example, a thirdparallel resonator can be added to the filter corresponding to FIG. 8 tosharpen steepness of skirt edge 806. As another example, resonancefrequency of the parallel resonator 702 can be tuned to frequency of theskirt edge 806, and sharpen the steepness of the skirt edge 806.

Configurations with more than two parallel resonators can be implementedaccording to the techniques described herein. FIG. 7C depicts an exampleband pass filter 720 that includes four parallel resonators 722, 724,726, and 728. The four resonators of the filter 720 are in parallel withthe LBAW 100. Similar to the resonators 702, 704, 712, each of theresonators 722-728 includes a piezo layer that is sandwiched between aresonator electrode and a resonator counter electrode. The piezo layer(e.g., piezo 110), the counter electrode (e.g., counter electrode 120),and/or the electrode of a resonator can be common between the resonatorand LBAW 100.

Resonators 724, 726 include a conductive layer 732, 734 that can becoupled to the input or the output electrodes of LBAW 100. Resonators722 and 728 includes one or more layers of mass loading on top of theirrespective conductive layer 732, 734. Resonator 728 includes a massloading layer 708 on top of the conductive layer 734. Resonator 722includes multiple mass loading layers 732 and 738 on top of theconductive layer 732.

The layers 732, 734, 708, 736, and 738 may be composed of the same ordifferent materials. For example, layer 732, 734 can be composed of Alor copper, and layer 708, 736, 738 can be composed of silicon oxide(SiO2), silicon nitride (SiN) and/or one or more metals. The layers 732,734, 708, 736, and 738 may have different thicknesses. In some examples,the conductive layer 732 may have a different thickness in resonator 722than in the resonator 724. In some examples, the conductive layer 734may a different thickness in resonator 726 than in resonator 728.

Depending on the properties (e.g., thickness, material) of the fourparallel resonators 722-728, the band pass filter 720 may act on theinsertion loss as an LBAW in parallel with four or less than fourresonators. When the four resonators have four different resonancefrequencies, insertion loss of the filter 720 is suppressed at the fourdifferent resonance frequencies. When two of the resonators have thesame resonance frequency, the insertion loss of the filter 720 issuppressed at three different frequencies because the resonators provideonly three different resonance frequencies. Similarly, if three or fourof the resonators have the same resonance frequency, the suppression ofinsertion loss happens only in two frequencies or one frequency,respectively.

For example, if resonators 724 and 726 have the same resonancefrequencies, the band pass filter 720 acts on the insertion loss as anLBAW with three parallel resonators (a), (b) and (c). The parallelresonator (a) is formed by the resonators 724 and 726, the parallelresonator (b) is formed by the resonator 728, and parallel resonator (c)is formed by the resonator 722. Since the two parallel resonators 724and 726 share the same resonance frequencies, they act as one resonator(i.e., the parallel resonator (a)) for purpose of suppressing thesideband. For example, if resonators 724 and 726 have the same size(e.g., the same thickness) and are composed of the same materials, theycan have the same resonance frequencies.

Each of the layers 732, 734, 708, 736, 738 of FIGS. 7A-7C can be made ofthe same or different materials as the LBAW extensions (e.g., extensions150 a or 170 a). In some embodiments electrodes 732, 734 and/or LBAWextensions are composed of Aluminum (Al), and the layers 708 and 736 arecomposed of silicon oxide (SiO₂) and/or silicon nitride (SiN). Forexample, layer 708 can be composed of SiO₂ and layer 736 can be composedof SiN. In some examples, a layer composed of SiO2 is less than 500 nmthick, and a layer composed of SiN is less than 100 nm thick. In someexamples, a layer composed of SiO₂ is 50 to 250 nm thick, and a layercomposed of SiN is 5-50 nm thick.

In some embodiments, one or more resonator electrodes are the uppermostlayers of the respective resonators. For example, layer 708 may be usedas an electrode of the resonator 712.

FIGS. 9A-B depict example resonators integrated into an LBAW filterthrough one or more connections. Filter 900 depicted in FIG. 9A cangenerally be any of filters 700, 710, or 720, except that it hasadditional resonator 914, 916 in parallel with LBAW 100. The resonators914 and 916 include electrodes 904 and 906, respectively, that areconnected to electrodes 908 through common edges 910 and 912,respectively. The electrodes 908 can be electrodes of LBAW 100, or canbe electrodes of one or more resonators (e.g., resonator 702, 704, 712,722, 724, 726, 728) in parallel with LBAW 100. The resonators 914 and916 can be electrically coupled to input or output electrodes of LBAW100, or to electrodes of one or more resonators in parallel with LBAW100. For example, the electrode 908 can be the conductive layer 734 andcan be electrically coupled to resonator electrode 906 by sharing thecommon edge 912. The resonators 914 and 916 share a common counterelectrode 120 (see FIG. 1A) with the LBAW 100. The resonators 914 and916 can also share a common piezo layer 110 with LBAW 100. The counterelectrode 120 can be grounded. Ground vias (or contacts) 902 are alsodepicted in FIG. 9A.

One or more parallel resonators can also be connected to an LBAW or toone or more other parallel resonators through one or more conductivelines. FIG. 9B depicts a conductive line 936 connecting a first parallelresonator 932 to a second parallel resonator 934. The first parallelresonator 932 has a first electrode 942, and the second parallelresonator has a second electrode 944. The first electrode 942 has afirst edge 922 facing the LBAW filter 100, a second edge 924 fartherfrom the LBAW filter, and a third edge connecting the first edge and thesecond edge. The second electrode 944 can be connected to any sides,edges, or corners of the first electrode 942. In the depicted example,the second electrode 944 is positioned adjacent to the third edge 926 ofthe first electrode 942. In some examples (not shown), the secondelectrode 944 can be positioned adjacent to the second edge 924 of thefirst electrode 942. In the depicted example, there is a gap between thefirst electrode 942 and the second electrode 944; the gap is bridges bythe conductive line 936. In some embodiments, the first and the secondelectrodes share a common edge (similar to the common edge 910 betweenelectrodes 908 and 904 in FIG. 9A). The first and the second parallelresonators 932 and 934 are electrically coupled through the firstelectrode 942 and the second electrode 944, for example, by sharing acommon edge, or a conductive line.

Electrodes of the first and/or second parallel resonators 932, 934 canbe loaded by one or more mass loading layers (e.g., layer 708, 736,738). The one or more mass loading layers can be selected so thatresonance frequency of the respective parallel resonator falls within asideband of LBAW 100. Sizes (e.g., thickness) of the first electrode942, the second electrode 944, and/or the conductive line 936 can beadjusted for tuning resonance frequencies of the respective resonatorsin LBAW 100's sideband.

FIG. 10 depicts another example of a filter 1000 including LBAW 100 inparallel with multiple resonators. Ground vias 1002 are also depicted inFIG. 10

Resonators 1004, 1006, 1008, and 1010 are in parallel with LBAW 100. Theresonators share a counter electrode 120 (see FIG. 1A) with LBAW 100.The counter electrode 120 can be grounded. Resonators 1004 and 1006share a conductive electrode layer 1018, and resonators 1008 and 1010share a conductive electrode layer 1026. Either or both of the layers1018 and 1026 can be electrically coupled with input and/or outputelectrodes of LBAW 100. In some examples, one or both layers 1018 and1026 can provide a common electrode between LBAW and one or moreresonators. Extensions of LBAW 100 project from the common electrode.Each of the layers 1018 and 1026 can be composed of metal, for example,Al.

Resonator 1004 includes a mass loading layer 1016 on top of theconductive electrode layer 1018. Layer 1016 covers a portion of layer1018. The layer 1016 can cover substantially all of the electrode layer1018 within the resonator 1004.

Resonator 1006 includes a mass loading layer 1014 on top of theelectrode layer 1018. The layer 1014 can be in form of a frame, e.g.,walls that are relatively thin compared to the region enclosed by thewalls. The frame can form a hollow polygon. The frame can extend alongthe perimeter of the resonator 1006. For example, in FIG. 9B, frame 938is formed (e.g., by deposition or etching) on top of electrode 932 andcan provide a frame-shape resonator. Further details about frame-shaperesonators can be found in PCT App. No. PCT/FI00/00591, entitled“Resonator Structure and a Filter Comprising Such a ResonatorStructure,” which is incorporated herein in its entirety.

Resonator 1008 includes a mass loading layer 1020 on top of theelectrode layer 1026. Similar to layer 1014, layer 1020 can be in formof a frame.

Resonator 1010 includes two mass loading layers 1022 and 1024 on top ofthe layer electrode 1026. Each of the mass loading layers 1022 and 1024can cover substantially all of the electrode layer 1026 within theresonator 1010. Layer 1022 may cover the whole, or part of the layer1024's surface.

Each of the mass loading layers 1014, 1016, 1020, 1022, and 1024 (eachcan also be referred to as a “load”) can be composed of metal orsemiconductor material. In the example filter 1000, layers 1014, 1016,1020, and 1024 are composed of SiO₂, and the layer 1022 is composed ofSiN. In some embodiments, thickness of SiO₂ layers are lower than 500nm, and thickness of SiN layers are lower than 100 nm. For example,thickness of SiO₂ layers can be 50 to 250 nm, and thickness of SiNlayers can be 5 to 50 nm.

In embodiments with multiple parallel resonators, two or more of theresonators can have electrodes of the same shape, or can have electrodesof different shapes (or sizes). For example, the conductive layer 732 inFIG. 7A can be circular and the conductive layer 734 can be rectangular.A resonator (e.g., one or both electrodes of the resonator) can alsohave a gradient thickness to spread out the shunting effect over a rangeof frequency.

The resonance frequency of a resonator can also be tuned by adjustinglateral shape of the resonator. Larger resonators (e.g., resonators withlarge electrode surfaces) have lower resistance at their resonancefrequencies, and provide a stronger shunting effect. Thus, to have astrong shunting effect, a large resonator with high Q (sharp responsepeak and low resistance) can be a desirable resonator for suppressingLBAW sideband.

If a resonator's width (e.g., a resonator electrode's width) is narrowerthan a threshold width, its resonance frequency starts to depend on theresonator's width. For such narrow resonators, the resonance frequencydepends on the width of the resonator, and the narrower the resonatoris, the higher its resonance frequency gets (assuming Type 2 dispersion,i.e. frequency increasing with decreasing lateral wavelength).

A resonator can be of any shape, such as circular, rectangular,doughnut, etc. For example, an electrode of LBAW can be in form of adoughnut and form a parallel resonator in doughnut shape. Impedance of anarrow doughnut resonator can peak at more than one resonance frequency,resulting in suppression of insertion loss over a wider range offrequencies compared to single resonance frequency resonators (e.g., arectangular resonator). It should be noted that in deciding the form ofa resonator, both resonance frequency and resistance of the resonator atthe resonance frequency should be considered. For example, compared to arectangular resonator a doughnut resonator may have multiple resonancefrequencies but higher resonance at each resonance frequencies,resulting in a milder shunting effect at the resonance frequencies.

A number of implementations have been described. Nevertheless, it willbe understood that various modifications may be made without departingfrom the spirit and scope of disclosure. Accordingly, otherimplementations are within the scope of the following claims.

What is claimed is:
 1. An acoustic wave filter device comprising: anacoustic wave filter element comprising interdigited input electrodesand output electrodes located on a top surface of a piezoelectric layerand a counter-electrode on a bottom surface of the piezoelectric layer;a first resonator coupled to the acoustic wave filter element andcomprising a first resonator electrode on the top surface of thepiezoelectric layer and a first resonator counter-electrode on thebottom surface of the piezoelectric layer, the first resonator having afirst notch in resonator impedance at a first frequency; and a secondresonator coupled to the acoustic wave filter element and comprising asecond resonator electrode on the top surface of the piezoelectriclayer, a second resonator counter-electrode on the bottom surface of thepiezoelectric layer, and a first mass loading layer on the secondresonator electrode such that the second resonator has a second notch inresonator impedance at a second frequency that is different from thefirst frequency.
 2. The device of claim 1, wherein the first frequencyand the second frequency are within a sideband of resonator impedance ofthe acoustic wave filter element.
 3. The device of claim 1, wherein thefirst frequency and second frequency differ by at least 3%.
 4. Thedevice of claim 1, wherein the first resonator electrode is electricallycoupled to the input electrodes, and the second resonator electrode iselectrically coupled to the output electrodes.
 5. The device of claim 1,wherein the first resonator electrode is electrically coupled to theinput or output electrodes, and the second resonator electrode iselectrically coupled to the first resonator electrode.
 6. The device ofclaim 5, wherein the first resonator electrode has a first edge facingthe acoustic wave filter element, the first resonator electrode has asecond edge on a side of the first resonator electrode farther from theacoustic wave filter element, and a third edge connecting the first edgeand the second edge.
 7. The device of claim 6, wherein the secondresonator electrode positioned adjacent the second edge of the firstresonator electrode.
 8. The device of claim 6, wherein the secondresonator electrode positioned adjacent the third edge of the firstresonator electrode.
 9. The device of claim 5, wherein the firstresonator electrode and second resonator electrode are electricallycoupled by sharing a common edge.
 10. The device of claim 5, wherein thefirst resonator electrode and second resonator electrode areelectrically coupled by a conductive line that bridges a gap between thefirst resonator electrode and second resonator electrode.
 11. The deviceof claim 1, comprising a third resonator coupled to the acoustic wavefilter element, the third resonator comprising a third resonatorelectrode on the top surface of the piezoelectric layer, a thirdresonator counter-electrode on the bottom surface of the piezoelectriclayer, the first mass loading layer on the third resonator electrode,and a second mass loading layer on the third resonator electrode suchthat the third resonator has a third notch in resonator impedance at athird frequency that is different from the first and second frequencies.12. The device of claim 11, wherein the second mass loading layer isdisposed on the first mass loading layer.
 13. The device of claim 12,wherein the first mass loading layer comprises a silicon oxide layer.14. The device of claim 13, wherein the second mass loading layercomprises a silicon nitride layer.
 15. The device of claim 11,comprising a fourth resonator coupled to the acoustic wave filterelement, the fourth resonator comprising a fourth resonator electrode onthe top surface of the piezoelectric layer, and a fourth resonatorcounter-electrode on the bottom surface of the piezoelectric layer. 16.The device of claim 15, wherein the fourth resonator has a fourth notchin resonator impedance at the first frequency.
 17. The device of claim1, wherein the first resonator electrode is an uppermost layer of thefirst resonator.
 18. The device of claim 1, wherein the first massloading layer does not cover the second resonator electrode.
 19. Thedevice of claim 1, wherein a first portion of the first mass loadinglayer is disposed on the first resonator electrode, a second portion ofthe first mass loading layer is disposed on the second resonatorelectrode, and the first portion and the second portion have differentthicknesses.
 20. The device of claim 1, wherein the counter-electrode ofthe acoustic wave filter element, the first resonator counter-electrode,and the second resonator counter-electrode are provided by a commoncounter-electrode that continuously spans the acoustic wave filterelement, first resonator and second resonator.
 21. The device of claim1, wherein the input electrodes, output electrodes, first resonatorelectrode, and second resonator electrode are provided by separateportions of the same electrode layer on the top surface of thepiezoelectric layer.
 22. The device of claim 1, wherein a thickness ofthe piezoelectric layer and a gap width between the input and outputelectrodes is such that application of a radio frequency voltage betweenthe input electrodes and the counter-electrode will create symmetric andantisymmetric acoustic thickness-extensional resonance modes in thepiezoelectric layer.
 23. The device of claim 1, wherein the acousticwave filter element is a laterally acoustically coupled bulk acousticwave (LBAW) filter.
 24. An acoustic wave filter device comprising: anacoustic wave filter element comprising interdigited input electrodesand output electrodes located on a top surface of a piezoelectric layer;a first resonator coupled to the acoustic wave filter element andcomprising a first resonator electrode on the top surface of thepiezoelectric layer, the first resonator having a first notch inresonator impedance at a first frequency; and a second resonator coupledto the acoustic wave filter element and comprising a second resonatorelectrode on the top surface of the piezoelectric layer, and a firstmass loading layer on the second resonator electrode such that thesecond resonator has a second notch in resonator impedance at a secondfrequency that is different from the first frequency.
 25. An acousticwave filter device comprising: an acoustic wave filter elementcomprising an electrode located on a top surface of a piezoelectriclayer and a counter-electrode located on a bottom surface of thepiezoelectric layer; a first resonator coupled to the acoustic wavefilter element and comprising a first resonator electrode on the topsurface of the piezoelectric layer and a first resonatorcounter-electrode on the bottom surface of the piezoelectric layer, thefirst resonator having a first notch in resonator impedance at a firstfrequency; and a second resonator coupled to the acoustic wave filterelement and comprising a second resonator electrode on the top surfaceof the piezoelectric layer, a second resonator counter-electrode on thebottom surface of the piezoelectric layer, and a first mass loadinglayer on the second resonator electrode such that the second resonatorhas a second notch in resonator impedance at a second frequency that isdifferent from the first frequency.